91Ó°ÊÓ

Carvone (Problem \(10.38\) ) has an intense infrared absorption at \(1690 \mathrm{~cm}^{-1}\). What kind of ketone does carvone contain?

Would you expect allene, \(\mathrm{H}_{2} \mathrm{C}=\mathrm{C}=\mathrm{CH}_{2}\), to show a UV absorption in the 200 to \(400 \mathrm{~nm}\) range? Explain.

Ergosterol, a precursor of vitamin \(\mathrm{D}\), has \(\lambda_{\max }=282 \mathrm{~nm}\) and molar absorptivity \(\epsilon=11,900 .\) What is the concentration of ergosterol in a solution whose absorbance \(A=0.065\) with a sample pathlength \(l=1.00 \mathrm{~cm} ?\)

Propose structures for compounds that meet the following descriptions: (a) An optically active compound \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}\) with an IR absorption at \(1730 \mathrm{~cm}^{-1}\) (b) An optically inactive compound \(\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{~N}\) with an IR absorption at \(2215 \mathrm{~cm}^{-1}\)

4-Methylpentan-2-one and 3-methylpentanal are isomers. Explain how you could tell them apart, both by mass spectrometry and by infrared spectroscopy.

Organomagnesium halides (R-Mg-X), called Grignard reagents, undergo a general and very useful reaction with ketones. Methylmagnesium bromide, for example, reacts with cyclohexanone to yield a product with the formula \(\mathrm{C}_{7} \mathrm{H}_{14} \mathrm{O}\). What is the structure of this product if it has an IR absorption at \(3400 \mathrm{~cm}^{-1}\) ?

Benzene has an ultraviolet absorption at \(\lambda_{\max }=204 \mathrm{~nm}\), and \(p\) -toluidine has \(\lambda_{\max }=235 \mathrm{~nm}\). How do you account for this difference?

Ketones undergo a reduction when treated with sodium borohydride, \(\mathrm{NaBH}_{4}\). What is the structure of the compound produced by reaction of butan-2-one with \(\mathrm{NaBH}_{4}\) if it has an IR absorption at \(3400 \mathrm{~cm}^{-1}\) and \(\mathrm{M}^{+}=74\) in the mass spectrum?

Nitriles, \(\mathrm{R}-\mathrm{C}=\mathrm{N}\), undergo a hydrolysis reaction when heated with aqueous acid. What is the structure of the compound produced by hydrolysis of propanenitrile, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{N}\), if it has IR absorptions at 2500 to \(3100 \mathrm{~cm}^{-1}\) and \(1710 \mathrm{~cm}^{-1}\) and has \(\mathrm{M}^{+}=74 ?\)

Enamines \((\mathrm{C}=\mathrm{C}-\mathrm{N} ;\) alkene \(+\) amine) typically have a UV absorption near \(\lambda_{\max }=230 \mathrm{~nm}\) and are much more nucleophilic than alkenes. Assuming the nitrogen atom is \(s p^{2}\) -hybridized, explain both the UV absorption and the nucleophilicity of enamines.